Chips for electronic devices, particularly sensors, can be made by stacking a cap on a substrate or wafer. In general, the cap may act as a protective shield for the active element or die. In the particular case of sensors, the cap may have additional uses and features, for instance as thermal insulator or as radiation filter. For particular embodiments of certain sensors, it is advantageous that the cap is comprised of silicon. In several further applications, a cavity is needed in the cap, which is usually etched.
The cap and the active element are fixed together, typically by wafer bonding, for example the cap is fixed to a sensor wafer. The cap adhesion is usually not sufficient, so a mold component is added for packaging purposes. The addition of this mold disadvantageously increases the device size. Despite the mold, the adhesion may still be lacking and delamination may take place, rendering the chip useless.
U.S. Pat. No. 7,575,953 discloses a three-die stack assembly in which one die is shaped for receiving another die, and the third die is shaped so there is room for connections in between the second and third die. This assembly can be fixed by adhesive or mold component, but the connections increase considerably the packaging size. In addition, the assembly is not optimal for certain type of devices, for instance radiation sensors.